Man-holes or air-shafts used by the public utilities, such as the telephone, power or gas companies, have collars of variable lengths and diameters. These holes or shafts are generally made of masonry or concrete and, independently of their length of service or location, their main drawback is cracks extending along the internal walls of their collars or under the lower ends thereof. These cracks, which are caused by the infiltration of rain, the erosion of the ground and/or the frost in winter, allow water to infiltrate around the collar and enter the man-hole. Of course, the water streams down inside the galleries and wets the main walking or working areas as well as the equipment that is located therein. Where electrical equipment is involved, the risk of injury due to short circuits and fires is very great.